So my Keyboardio Model 100 does not work with Grub (ie: the arrow keys and enter key have no effect). I tried putting the keyboard in boot mode with ( Left Fn + Esc + Shift ) but that had no apparent affect.
I updated the firmware to the latest (0.91.1+74) and have Chrysalis 0.13.2.
This might sound funny, but you could try with a different USB port.
One of my old keyboards didn’t seem to work properly in Grub due to what I believe was too high key repeat rate on USB 3.0. After I’d pressed the first key Grub seemingly got stuck and would not respond to further key presses. There’s probably a better fix for this (BIOS?), but switching to a different (presumably slower) USB port took care of this and made it usable.
Personally I have no problems using the model 100 with Grub (with the one port I have tried with at least).